As is well known, mass flow sensors are suitable for measuring the mass flow rate and/or the density of fluids flowing in a conduit in which the mass flow sensor is installed.
Coriolis-type mass flow sensors with at least two measuring tubes can be better made immune to disturbances originating from the conduit, such as vibrations of the conduit or wide pressure variations of the fluid, than Coriolis-type mass flow sensors having only a single measuring tube, which are also more sensitive to other disturbances acting on them, such as impacts on the casing.
These disturbances also include vibration components which are transmitted from the measuring tube vibrating at an instantaneous frequency to the conduit and are reflected from discontinuities in the conduit, thus traveling back to the measuring tube. These vibration components are particularly disturbing because they have the same instantaneous frequency as the measuring tube. Therefore, these disturbances cannot be eliminated by electronic means and are difficult to eliminate by mechanical means.
U.S. Pat. No. 5,307,689 describes two variants of Coriolis-type mass flowmeters whereby an attempt is made to eliminate the above-mentioned conduit-induced disturbances by circuit means, but the components having the same frequency as the vibration of the measuring tube cannot be eliminated. The design of the Coriolis-type mass flow sensor do not show any measures to suppress disturbances.
DE-A 38 24 351 describes a Coriolis-type mass flow sensor in which the rigid connection between two measuring tubes and a casing, which is stiff against vibration, is claimed to keep the above-mentioned external disturbances away from the two measuring tubes. It has turned out, however, that sufficient suppression of external disturbances cannot be achieved in this manner.
U.S. Pat. No. 5,705,754 describes a Coriolis-type mass flow sensor which can be installed in a conduit and which during operation is traversed by a fluid to be measured, comprising:
a casing to be connected with the conduit via a fluid inlet and a fluid outlet; PA1 a support base disposed within the casing, PA1 a single measuring tube, traversed by the fluid, which is bent in a plane parallel to a top side of the support base and ends in a fluid inlet and a fluid outlet, PA1 a vibrator for producing the vibrations; and PA1 a first sensing element mounted near the first fixing means for sensing the vibrations of the tube portion as well as a second sensing element mounted near the second fixing means for sensing the vibrations of the tube portion. PA1 a casing to be connected with the conduit via a fluid inlet and a fluid outlet; PA1 a rigid support base disposed within the casing, PA1 a measuring tube traversed by the fluid and ending in the fluid inlet and the fluid outlet, PA1 a vibrator arrangement for vibrating the tube portion, said vibrator assembly acting between the casing and the support base; PA1 a first sensing element mounted near the inlet-side fixing means, partly on the tube portion, for sensing the vibrations of the tube portion; and PA1 a second sensing element mounted near the outlet-side fixing means, partly on the tube portion, for sensing the vibrations of the tube portion.
said support base being connected to the casing via at least one mechanical damping element PA2 a portion of said measuring tube to be vibrated in a direction perpendicular to said plane being fixed to the support base by a first fixing means provided in the area of a first edge of the support base and by a second fixing means provided in the area of a second edge of the support base, and PA2 a first connecting portion of said measuring tube extending from the first fixing means to the fluid inlet, and a second connecting portion extending from the second fixing means to the fluid outlet; PA2 said support base being connected with the casing via at least one mechanical damping element; PA2 a portion of said measuring tube to be vibrated being fixed to the support base by an inlet-side fixing means and an outlet-side fixing means, and PA2 an inlet-side connecting portion of said measuring tube extending from the inlet-side fixing means to the fluid inlet, and an outlet-side connecting portion extending from the outlet-side fixing means to the fluid outlet;
which is located on a nodal line of vibration of the support base;